Xerographic toners of a resin, a pigment, and a charge control agent are known. Toners useful for xerographic applications should exhibit certain performances related to storage stability, and particle-size integrity; that is, the toner particles should remain intact and not agglomerate until fused on paper. The toner compositions also should not substantially agglomerate at temperatures below about 50° C. to about 55° C., because environmental conditions vary. The toner compositions should also display acceptable triboelectric properties that vary with the type of carrier or developer composition.
It is also desirable for xerographic toner compositions to have low-temperature fusing on paper. There is pressure to reduce the fusing or fixing temperatures of toners onto paper, for example, to temperatures of from about 90° to about 120° C., to lower power consumption and to allow extended fuser-system lifetimes. Non-contact fusers, which heat toner images on paper by radiant heat, usually are not in contact with the paper and the toner image. Contact fusers, on the other hand, are in contact with the paper and the toner image, and the toner compositions used with contact fusers should not substantially transfer onto the fuser roller.
Toner-fixing performance can be characterized as a function of temperature. The maximum temperature at which the toner does not adhere to the fuser roll is called the hot-offset temperature (HOT). When the fuser temperature exceeds the toner's HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images. This transfer may result in blurred images. This undesirable phenomenon is called hot offset or cold offset depending on whether the temperature is below the fixing temperature of the paper (cold offset), or above the fixing temperature of the toner (hot offset).
The minimum fixing temperature (MFT) of the toner, which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, should be as high as possible, but is always less than the toner composition's HOT. The MFT is determined, for example, by a crease test. The difference between MFT and HOT is called the fusing latitude of the toner, i.e., the temperature difference between the fixing temperature and the temperature at which the toner offsets onto the fuser.
Additionally, small-sized toner particles, such as those having average particle sizes of from about 3 to about 12 microns, such as from about 5 to about 7 microns, are desired, especially for use in high-resolution xerographic engines. Small-sized toner particles can be economically prepared by chemical processes, which involve the direct conversion of emulsion-sized particles to toner composites by aggregation and coalescence, or by suspension, micro-suspension, or micro-encapsulation processes.
Low-temperature-fixing toners comprised of semi-crystalline resins are known. For example, U.S. Pat. No. 5,166,026 discloses semi-crystalline copolymer resin toners, with melting points of from about 30° C. to about 100° C., and containing functional groups comprising hydroxy, carboxy, amino, amido, ammonium or halo, and pigment particles. Similarly, U.S. Pat. No. 4,952,477 discloses toner compositions of semi-crystalline polyolefin resin particles, with melting points of from about 50° C. to about 100° C., and containing functional groups comprising hydroxy, carboxy, amino, amido, ammonium or halo, and pigment particles. Although, some of these toners may provide low contact fixing temperatures of about 93.3° C. to about 107.2° C., the resins are derived from components with melting characteristics of about 30° C. to about 50° C., and are not believed to exhibit higher, more desirable melting characteristics, such as about 55° C. to about 60° C.
Crystalline-based toners are disclosed, for example in U.S. Pat. No. 4,254,207. Low-temperature-fixing toners comprised of cross-linked crystalline resin and amorphous polyester resin are illustrated in U.S. Pat. No. 4,990,424, in which the toner powder is comprised, for example, of polymer particles of partially carboxylated crystalline polyester and partially carboxylated amorphous polyester that has been cross-linked together at elevated temperature with the aid of an epoxy resin and a cross-linking catalyst.
Conventional low-melt toner compositions, such as those described above, generally comprise from about 10 to about 35% of an unsaturated crystalline resin and from about 90 to about 65% of a branched, amorphous polyester resin. Such toner compositions meet the crease, gloss, latitude, and charging performance requirements of high-speed production printing. These toners also meet heat-cohesion requirements when less than 10% additives are present. Such toners are prepared by conventional melt-extrusion techniques. However, the crystalline components of such toners are very ductile and are difficult to reduce to small particles, such as particles having an average particle diameter of about 7 microns, in sufficiently high yields.
There is thus a need to provide low-melt toners that include crystalline and amorphous polyester resins, that may be provided as small particles in high yields, and that may be used at lower fusing temperatures that still provide excellent properties including excellent document offset and heat cohesion, for good image quality, particularly for color copies and prints. There is also a need to provide economical processes for preparing such low melt toners that allow for controlled particle growth and controlled morphology or shape, and provide high yields of small particles.